This invention relates generally to acoustic wave devices, and more particularly, to substrate materials for use in surface acoustic wave (SAW) devices. These devices employ substrates of a piezo-electric material, across which elastic surface waves are propagated between sets of electro-acoustic transducers disposed on the substrate surface. The devices employ so-called Rayleigh waves, which can be propagated along a free surface of a solid, and have an amplitude of displacement that is largest right at the substrate surface. In a piezoelectric material, deformations produced by such waves induce local electric fields, which are propagated with the acoustic waves and extend into space above the surface of the material. These electric fields will interact with electrodes disposed on the surface of the material, to serve as electrical input and output transducers for the surface acoustic wave device.
Substrates for SAW devices are usually highly anisotropic in nature, i.e. the velocity of wave propagation varies strongly with the direction of propagation. SAW devices, therefore, usually employ a single direction of wave propagation.
It is well known that surface acoustic waves behave analogously to light waves in many respects. In particular, interference and diffraction effects in optics have counterparts in surface acoustic wave technology. However, SAW devices employing principles of interference or diffraction are required to transmit waves in more than one direction. If the SAW substrate is anisotropic, the only way to implement interference or diffraction devices in SAW technology is by compensating for the velocity differences by appropriate positioning of the transducers. This approach is used, for example, in apparatus disclosed in a copending application of Robert E. Brooks, Ser. No. 529,066, filed on Sept. 2, 1983, and entitled "Signal Processing System and Method."
If diffraction and interference effects can be exploited directly in SAW devices, powerful signal processing functions may be implemented, such as spectrum analysis and Fourier transformation. Ideally, however, these functions require almost perfectly isotropic substrates, to permit the use of transducer patterns that are acoustically correct for a wide range of propagation directions at each point on the surface. Lead zirconium titanate (PZT) and zinc oxide (ZnO) are horizontally isotropic materials that have been available for this purpose, but their performance at high frequencies, above 60 megahertz (MHz), is poor. Trigonal materials that are in most respects highly suitable for use in SAW devices, such as quartz and lithium niobate (LiNbO.sub.3), have long been known to be highly anisotropic in nature, and hence not suitable for devices employing diffraction or interference principles. Specific cuts of lithium niobate crystals have been recognized as being less likely to propagate bulk shear waves. For example, U.S. Pat. No. 4,409,571 to Milson et al. is concerned with selection of lithium niobate to minimize bulk waves.
It will be appreciated from the foregoing that there is a need for a high-quality isotropic SAW substrate, having low propagation loss, low cost, a high coupling coefficient, and excellent uniformity. Preferably, these desirable properties should also be obtainable at high frequencies. The present invention fulfills this need.